Creation of ionized particles is a useful tool for many applications, such as for ignition of lasing or to assist chemical analysis, among other uses. In some equipment, high energy radioactive sources of alpha or beta particles are employed for the ionization process. However, because of the potential health hazard and need for regulation, wide-spread use of equipment using radioactive ionization sources has been limited. And even though smoke alarms use radioactive sources, the amount of ionization is low, and they still require government regulation.
There are several ionization methods that avoid radioactive sources. Corona discharge is a source of non-radioactive ionization. It provides high energy in a compact package. However, this process is not stable and can contaminate the sample with metal ions or NOx, as would interfere with analytical results. Furthermore, the generated ion species depends upon the applied voltage.
RF discharge ionization reduces some of these disadvantageous effects. RF discharges are subdivided into inductive and capacitive discharges, differing in the way the discharge is produced.
Inductive methods are based on electromagnetic induction so that the created electric field is a vortex field with closed lines of force. Inductive methods are used for high-power discharges, such as for production of refractory materials, abrasive powders, and the like.
Capacitive discharge methods are used to maintain RF discharges at moderate pressures p˜1–100 Torr and at low pressures p˜10−3–1 Torr. The plasma in them is weakly ionized and non-equilibrium, like that of a corona discharge. Moderate-pressure discharges have found application in laser technology to excite CO2 lasers, while low-pressure discharges are used for ion treatment of materials and in other plasma technologies.
Another ionization process is UV ionization. This process is sometimes referred to as atmospheric pressure photo-ionization (APPI). In low pressure conditions, photo-ionization involves direct interaction of photons with samples, forming positively charged molecular ions and free electrons. At elevated pressure conditions, the situation is not so simple and the ionization process for sample molecules can include a sequence of gas phase reactions, the details of which depend on the energetic properties of initially formed ions and free electrons (due to direct photo-ionization) and on the nature of the ambient gas.
One disadvantage of UV ionization is that it provides low to moderate ionization energies. This limits the types of molecules that can be ionized. As well, sometimes APPI can give unexpected results. The photons are typically generated in a tube, with the photons passing through a window, and this window material affects efficiency. Also, the surfaces of the UV devices can become contaminated or coated from the ionization product, which can degrade device performance or output intensity. As well, the UV tubes can be delicate and fragile, and hence are generally not suitable to operation in harsh environments or in applications requiring a significant amount of manual handling.